So, this is sort of based off my previous question, What Would Be The Effect of Avatars In The Military?. After the fantasy society specified in this question developed into the twenty-first century, society fell to pieces, some idiot got out the nukes, and the result was an apocalypse.
A thousand or so years later, the descendants of the humans who took to space or went into heavy-duty bomb shelters emerged and found creatures they named Kaijura, the result of interbreeding and mutation between Avatars (being made of magic, they were destroyed by radiation, but the ghosts of all the living things who died provided the life-force and Ether necessary for them to reform).
During humanity's long period of isolation, they lost the secrets of creating Avatars, and of magic in general (if magic was left to the people, they would have figured out how to create Avatars again, and the ones in charge didn't want that for space/control of the populace reasons). So, they have no idea that these Kaijura are (sort of) descended from human beings.
The Kaijura for their part lost the knowledge of what they came from (Avatars) and the connection between Avatar and man. Humanity became the stuff of legend for them. However, after humanity returned, some Kaijura ended up forming a bond within humans, reawakening the dormant Avatar traits (telepathic link and magical healing, synergistic in this case) within them. This bond also made Kaijura stronger, making them capable of unleashing more powerful attacks, and it came with other benefits (companionship, survival, and so on).
This explains why most Kaijura would fight for humans in ritualistic battles (think Pokemon); it would be to prove their power, gain status, and train to defend themselves (people spar to do that, yes?). However, what about humanoid Kaijura? Some examples would be:
Lotusprite-Looks like a three or four-year-old girl with wings like lotus petals, pointed ears, pixyish features, and clothing made of leaves and flowers (perhaps grown from the skin?). Basically a fairy; evolves into Elvaris.
Misseena-Similar to Lotusprite in terms of age, but has a fishtail, fins, gills, and scales. Basically a mermaid; evolves into Samurin or Samurayna (ice or fire mermaid).
Coaldier-The size is proportional to a four or five-year-old, but with the musculature of a physically fit adult (male or female, doesn't matter, shape and musculature must be above average, say that of a weightlifter?). No one knows what Coaldier actually looks like, it is clad in bulky coal-black armor that only leaves the eyes exposed. (If you want an idea of what the armor looks like, look up Shardplate). Coaldier evolves into Coalanx, then into Anthrasmite.
These Kaijura are essentially human, with cultures and ideals of their own. Why would they let a human guide them, even obey orders from one, and leave their home and their society to partner with a human from a society alien to them? Restated, my question is: Why Would Humanoid Kaijura Partner With Humans?
For more information on Avatars, please read the linked question. As always, I appreciate your input, and your feedback. Thank you all!